The present invention relates to a spark plug available as an ignition device for internal combustion engine, and more specifically to a spark plug capable of promoting rupture of fuel bridge if it should occur so as to fill a spark discharge gap, to thereby successfully suppress degradation in the ignition property.
Conventional spark plug generally comprises a center electrode protruded downward from the end face of an insulator, and a ground electrode joined at one end thereof to a metal shell, and is composed so as to form a spark discharge gap between the end face of the center electrode and ground electrode, where electric spark generated in the gap ignites mixed fuel gas. To improve startability of the spark plug at low temperature, it has been a general practice for internal combustion engine to raise concentration of fuel-air mixture sucked into a combustion chamber.
Suction of a fuel-air mixture of higher concentration aiming at improving the startability at lower temperature, however, tends to cause accumulation of the fuel in a liquid state within pistons. The accumulated fuel may adhere on the surface of the spark plug or fill the spark discharge gap in conjunction with reciprocating motion of the pistons at the starting, which results in formation of fuel bridge at the spark discharge gap. Since the fuel is electro-conductive, such formation of fuel bridge at the spark discharge gap will be causative of leakage of current even if a high voltage is applied to the spark discharge gap, and will prevent electric spark from being generated at the spark discharge gap. The fuel-air mixture after sucked into a combustion chamber will therefore not be ignited, which undesirably degrade the startability contrary to expectation.
It is therefore an object of the present invention to provide a spark plug which is less causative of the fuel bridge at the spark discharge gap even when the above-described, hyper-concentration, fuel-air mixture is supplied.
The present invention relates to a spark plug which comprises an insulator (1) having a center through hole (1D); a center electrode (2) disposed in the center through hole (1D) and extends along the direction of axial line (O); a metal shell (5) having a screw (5B) for assembling an internal combustion engine provided external of the insulator (1); and a ground electrode (11) joined at one end thereof through a joint portion (55) to the metal shell (5), and on the other end of which having a discharge plane (111A) being arranged so as to oppose to an end face (22B) of the center electrode (2) to thereby form a spark discharge gap (g); and which spark plug is characterized,
wherein the insulator (1) is engaged to the metal shell (5) through an engagement portion (15), and the center electrode (2) is protruded out from an end face (1E) of the insulator (1) and has formed therein a divergent portion (G), having a diameter increasing towards the end, on the end side beyond such engagement portion (15) and between the outer peripheral surface of the center electrode (2) and the inner peripheral surface of the insulator (1); Moreover, the end of the center electrode (2) forming the spark discharge gap (g) comprises a noble metal member (22) having a straight rod portion (22A) of 1.0 mm or less in diameter and 0.2 mm or more in length; and
assuming a primary intersectional line (PKL) as being defined as an intersectional line formed between the end face (22B) of the center electrode (2) or a plane (P1) extended therefrom and a lateral plane (22S) of the straight rod portion (22A) or a cylindrical plane (C1) extended therefrom;
further assuming a secondary intersectional line (SKL) as being defined as an intersectional line formed between the discharge plane (111A) or a plane (P2) extended therefrom and an end face (112B) of the ground electrode (11) or a plane extended therefrom;
further assuming a primary virtual line (PVL) as being defined as a virtual line containing a primary intersectional point (PP) and being in parallel to a virtual center axial line (O) of the spark plug referring to the screw (5B) for assembling the internal combustion engine,
wherein the primary intersectional point (PP) is a first point encountered the primary intersectional line (PKL)when a standard line (SL) parallel to the virtual center axial line (O) is moved across the spark discharge gap (g) to the joint portion 55 of the ground electrode (11) from the side opposite to such joint portion (55) placing the virtual center axial line (O) in between; and
further assuming a secondary virtual line (SVL) as being defined as a virtual line containing a secondary intersectional point (SP) and being in parallel to the virtual center axial line (O),
wherein the secondary intersectional point (SP) is a last point where the standard line (SL) similarly moved intersects with the primary intersectional line (PKL);
a width of coverage (K) as being defined as a distance between the primary virtual line (PVL) and secondary intersectional line (SKL) is set so as to satisfy a relation of
xe2x88x92dxe2x89xa6Kxe2x89xa60.5 (in mm)xe2x80x83xe2x80x83(1)
(in mm: where d represents the diameter of the end face (22B) of the center electrode (2); and sign for K is defined as negative when the secondary intersectional line (SKL) stands closer to the joint portion (55) than the primary virtual line (PVL), and as positive when stands further); and
the width (w) of a portion of the discharge plane (111A) which falls within a range (WDS) between the secondary virtual line (SVL) and primary virtual line (PVL) satisfies a relation of
w less than 2.1xe2x88x92K (in mm)xe2x80x83xe2x80x83(2)
where K is the foregoing width of coverage.
It should now be noted that reference numerals and alphabets assigned to the individual constituents given in the Claims of the invention and in this section (SUMMARY OF THE INVENTION) were quoted from those used for the corresponded constituents shown in the attached drawings (FIGS. 1, 2 and 12), which are merely for the purpose of facilitating understanding of the present invention, and by no means limit the concept of the individual constituents in the present invention.
According to the foregoing constitution, the fuel bridge is likely to rupture even if it should occur at the spark discharge gap, since areas for the fuel contact on the ground electrode and center electrode are reduced. More specifically, at the beginning of the operation, a starter motor cranks to allow admission of fuel-air mixture into a combustion chamber. Although use of a hyper-concentrated fuel-air mixture inevitably causes the fuel bridge at the spark discharge gap in conjunction with the motion of the pistons at the start time, the fuel bridge in the spark plug of the present invention is likely to rupture by vibration applied when the cranking is further sustained.
The spark plug is generally attached to an internal combustion engine so as to direct the side of the spark discharge gap downward. The fuel bridge generated at the spark discharge gap is sustained so that the liquid droplet of the fuel is suspended by adhesive force effected between such liquid droplet and the center electrode. Since the spark plug is designed so as to reduce the diameter of the end of the center electrode as small as 1.0 mm or less, which reduces an area for retaining the fuel droplet, so that the bridge will readily be ruptured even if it should undesirably be formed. It is also worth while pointing out that the center electrode has a straight rod portion of 0.2 mm or above in length, and that the rear side of such portion is connected to a divergent portion of the center electrode. Since the fuel bridge once formed with a hyper-concentrated fuel-air mixture extends over the side face of the center electrode, so that the elongation of the straight rod portion is advantageous in that preventing the fuel from spreading over a transitional portion towards the divergent portion. This successfully downsizes the area for retaining the fuel droplet and thus reduces retention force effected between the center electrode and liquid droplet, which makes the fuel bridge more likely to rupture. In addition, composing the end portion of the center electrode with a noble metal will desirably suppress the wear due to spark discharge, which makes it possible to suppress deformation due to the wear during long-term use, and to retain the easiness in rupture of fuel bridge for a long period. Noble metals exemplified herein include not only Pt and Ir, but also those having a melting point of 1,600xc2x0 C. or above such as Pt alloys and Ir alloys which are typified by Ptxe2x80x94Ir, Irxe2x80x94Rh, Irxe2x80x94Pt, and Irxe2x80x94Y2O3.
The width of coverage K can be measured using a projector (as typically shown in FIG. 2B, measured based on a projection onto a projection plane in parallel both to a direction of the spark discharge gap (g) as seen from the joint portion (55) of the ground electrode (11) and to the center axis O). The outer peripheries of some discharge planes may be rounded or chamfered. For this case, an intersectional line formed by planes extended from the discharge plane and extended from the side face of the discharge-plane-forming portion (base member for the ground electrode or the protruded portion made of a noble metal) will serve as a boundary line based on which the width of discharge plane is discussed. In some other cases, a burr ascribable to cutting of the noble metal member may protrude into a portion of the primary intersectional line. For such case, the primary intersectional line must be imaged assuming that the burr has removed. Further for the case in which a rectangular wire is cut at predetermined intervals along the longitudinal direction to thereby produce the ground electrode, thus-produced cut plane which serves as an end face of the ground electrode may have steps ascribable to the cutting. For this case, it is to be defined that the secondary intersectional line is set on the basis of the end face closest to the discharge plane.
In the spark plug of the present invention, the width of coverage (K), defined as a distance between the primary virtual line (PVL) and secondary intersectional line (SKL) is set so as to satisfy the foregoing formula {circle around (1)}, which expresses xe2x88x92dxe2x89xa6Kxe2x89xa60.5 . The width of coverage K corresponds to a distance between the end face of the ground electrode and a virtual line (primary virtual line (PVL)) drawn at the position furthest from the joint portion with the ground electrode to the outer periphery of the end face of the center electrode in the axial direction. It is also to be defined that the width W of a portion of said discharge plane (111A) which falls within a range (WDS) between the secondary virtual line (SVL) and primary virtual line (PVL) is set so as to satisfy the foregoing formula {circle around (2)}, which expresses w less than 2.1xe2x88x92K.
If the width of coverage K falls less than xe2x88x92d, the side face of the end portion of the center electrode will oppose to the end face of the ground electrode. Such constitution is disadvantageous in reducing the area from which the fuel droplet suspends, since the length of the straight rod portion which composes the end portion of the center electrode must be excessively long, and the divergent portion which extends from the straight rod portion must be narrow. This adversely affects the heat radiation from the straight rod portion and tends to promote the wear thereof due to spark discharge. And what is more, the tendency of the wear due to spark discharge is strong, since the area of the end face of the ground electrode cannot be set as to be so large. In the present invention, the width of coverage K is however set to xe2x88x92d or above so as to oppose the end face of the center electrode to the discharge plane of the ground electrode. This allows the fuel bridge to be formed only within a spark discharge gap between the end face of the center electrode and the discharge plane of the ground electrode, which is advantageous to avoid the foregoing failure. It is recommendable for this case that the portion around the end face of the center electrode and the discharge plane of the ground electrode have morphology capable of facilitating rupture of the fuel bridge.
On the other hand, the width of coverage K is set as 0.5 mm or less, and the width w of a range obtained by extending, along the direction of the axial line, the end face of a portion of the discharge plane of the ground electrode which falls within a range between the primary virtual line PVL and secondary virtual line SVL is limited to less than (2.1xe2x88x92K) mm, which successfully reduces the area on the side of the ground electrode on which the fuel droplet is retained during formation of the fuel bridge. Since the ground electrode supports the fuel droplet from the bottom thereof, reduction in the supporting area means reduction in supportable volume of the fuel droplet. This successfully allows the fuel bridge, if it should occur, to readily be ruptured by repetitive vibration. It should be noted now that retaining ability of the fuel droplet increases if the width of coverage K is large enough even if the width w of the ground electrode remains unchanged. The formula {circle around (1)} thus means that the larger the width of coverage K grows, the smaller the upper limit value of the discharge plane width w should be in order to suppress formation of the fuel bridge. Conversely saying, this also means that the fuel bridge does not tend to occur even if the discharge plane width w grows somewhat larger provided that the width of coverage K is kept small.
Such dimensional setting of the width of coverage K can also improve the ignition property. One factor largely affects the ignition property relates to quenching effect by the electrode. Even if the fuel-air mixture is once ignited by electric spark generated in the spark discharge gap, the electrode which resides in the vicinity of the ignited fuel-air mixture takes the heat away, which results in flame-out of the fuel-air mixture. In contrast, reducing the width of coverage as in the present invention can expel the electrode which is causative of the flame-out from the area containing the fuel-air mixture, which improves the ignition property, and further improves the startability at lower temperature. It will be more advantageous to compose the protruded portion of the ground electrode by joining rectangular small members as described later, which is convenient to reduce the width of coverage K.
Besides the reduction of quenching effect, another advantage relates to that it will not disturb the flame diffusion as described below. The fuel-air mixture once ignited as described in the above can diffuse in the combustion chamber. This allows the entire fuel-air mixture in the combustion chamber to combust to thereby obtain larger output with an improved efficiency. A large width of coverage K herein means that the ground electrode can act as a screen to thereby obstruct the diffusion, in the early stage thereof, of the fuel-air mixture ignited in the spark discharge gap into the combustion chamber. On the contrary, a width of coverage K exceeding 0.5 mm herein may undesirably accelerate wear of the ground electrode due to overheat.
An excessively small discharge plane width w may sometimes accelerate wear of the electrode due to an excessive voltage concentration on the discharge plane to thereby make it difficult to sustain a desirable lifetime of the electrode, so that the width w is preferably ensured typically at 0.5 mm or above. The discharge plane width w is more preferably set so as to satisfy a relation of 0.5xe2x89xa6w less than 1.7xe2x88x92K (in mm).
Next strategy relates to that the ground electrode (11) can have formed thereon, at a position opposed to the end face (22B) of the center electrode (2), a rectangular protruded portion (112) protruded from the surface (111A) of the base member, which composes the discharge plane of such ground electrode (11), towards the center electrode (2). Provision of such protruded portion on the surface (111A) of the base member of the ground electrode successfully restricts a portion on which the fuel droplet is likely to be retained only within an area close to the protruded portion. The volume of the fuel droplet possibly retained on the side of the ground electrode can thus be reduced, which more effectively prevents the fuel bridge from being formed. In order to enhance the foregoing effect, it is preferable that the protruded portion (112) is protruded 0.5 mm or more from the surface (111A) of the base member of the ground electrode.
It is also preferable that the area of the end face (112A) of the protruded portion (112) is larger than that of the end face (22B) of the center electrode (2). The fuel bridge can be ruptured only when the gravity effecting on the fuel droplet overwhelms the adhesive force for maintaining the bridge formation (for example, boundary tension between the droplet and the individual end faces). If the area of the end face of the protruded portion is smaller than that of the center electrode, the adhesive force, which is expressed between the center electrode and droplet when the fuel bridge is formed, will exceed the gravity effected to such droplet, which may make it difficult to rupture the fuel bridge. On the contrary, ensuring a larger area of the end face of the protruded portion than that of the center electrode will desirably avoid such nonconformity.
The protruded portion (112) can be composed of a noble metal member. The ground electrode generally kept at a potential higher than that of the center electrode can attract light-weight electrons when electric spark generates. The ground electrode will thus have only a limited range of wear, but is likely to be heated as compared to the center electrode since it is located more closer to the center of the combustion chamber, and may suffer from accelerated wear depending on the types of internal combustion engines. Composing the protruded portion which composes the discharge plane of the ground electrode with a noble metal member less likely to be worn can successfully suppress the deformation-by-wear of the protruded portion, and can ensure easy rupture of the fuel bridge over a long period. Noble metals available herein are similar to those composing the center electrode, which include not only Pt and Ir, but also those having a melting point of 1,600xc2x0 C. or above such as Pt alloys and Ir alloys which are typified by Ptxe2x80x94Ir, Irxe2x80x94Rh, Irxe2x80x94Pt, and Irxe2x80x94Y2O3.
According to the spark plug of the present invention, the insulator (1) can be engaged to the metal shell (5) through the engagement portion (15) so as to protrude the center electrode (2) out from the end face of the insulator (1). In this case, the spark plug is composed so as to form a divergent portion (G), having a diameter increasing towards the end, on the end side beyond the engagement portion (15) and between the outer peripheral surface of the center electrode (2) and the inner peripheral surface of the insulator (1).
Such constitution successfully ensures a large difference in diameter between the center electrode and the end portion of the insulator. As described in the above, the liquid-state fuel retained in the piston will be flung up in association with motion of the piston, and will be transferred to the spark plug. More specifically, the fuel is supplied to the ignition portion of the spark plug shown in FIG. 2B from the bottom side of the drawing. If the fuel is charged in a large amount, it adheres to the entire space formed between the end portion of the insulator and the ground electrode. When the cranking is sustained thereafter, the generated vibration will cause drop-off of the adhered fuel from the outermost side of the end portion of the insulator. When a large difference in diameter between the center electrode and the end portion of the insulator is ensured, such diameter differing portion can retain a large volume of fuel, which allows the fuel to readily drop as being affected by vibration in the cranking. Rupture of the fuel bridge is thus promoted since the fuel adhered at the end portion of the insulator can readily drop in the early stage of the cranking as described in the above.
The divergent portion (G) may be formed so that the diameter continuously increases along the axial direction thereof, or increases in a step-wise manner in two or more steps. Even for the case of step-wise increase overall, continuously increase partially in a midway section is also allowable. A method for generating difference in diameter may be any of those such that reducing the diameter of the end portion of the center electrode towards the end side, such that increasing the diameter of the through hole of the insulator in which the center electrode is inserted, and combination of these methods.